1. Field
The present invention relates to a semiconductor optical amplifying device, a semiconductor optical amplifying system and a semiconductor optical integrated element being polarization-independent having high saturation optical output power.
2. Description Of The Related Art
In order to deal with a dramatic increase in demands for communication in recent years, introduction of so-called wavelength division multiplexing (WDM) system, which realizes high-capacity transmission by a single optical fiber by way of multiplexing plural signal lights of different wavelengths, shows progress. In the WDM system, an optical power of optical signals attenuates due to a loss of respective optical parts which are used for multiplexing or demultiplexing the optical signals differed in wavelengths, so that it is indispensable to use an optical amplifier to compensate for the attenuation.
Since a semiconductor optical amplifier (SOA) is small in size, and can be designed to have a gain in a wide range of wavelength, it is expected as an optical amplifier for compensating for the loss in the WDM system.
An optical fiber used for a high-capacity optical communication system does not maintain a polarization mode of optical signals, so that the SOA is required to have a gain difference between polarizations kept at a low value. Further, when the SOA is used in a saturation region, a transmission penalty is caused because of a waveform deterioration due to a pattern effect, and a cross talk between wavelength channels, so that it is needed to use the SOA in a linear region by sufficiently enhancing saturation optical output power.
For the SOA simultaneously realizing the small gain difference between the polarizations and the high saturation optical output power as described above, the following various characteristics are required.    Requirement A: An optical gain of the SOA does not change depending on a polarization state of an input signal light. Specifically, the SOA has a small gain difference between the polarizations.    Requirement B: A gain variation of the SOA due to the wavelength of the input signal light is small. Specifically, the SOA has a flat gain spectrum.    Requirement C: A signal-to-noise ratio deterioration of the signal light caused by interposing the SOA is small. Specifically, the SOA has a small noise figure.    Requirement D: The signal light can be amplified to sufficiently high output power by SOA. Specifically, the SOA has a high saturation optical output power.
In order to introduce the SOA into the optical communication system, the SOA is needed to satisfy all the requirements A to D. Further, the characteristics of the SOA are preferable to be obtained in as a wide wavelength region as possible of 1.55 μm band which is a wavelength used especially in a long-distance communication system. Hitherto, an SOA technology fulfilling the above four requirements is developed, in which an SOA having high output power, being polarization-independent and having low noise in the wavelength band of 1.55 μm is reported (for instance, refer to Non-Patent Documents 1 and 2).
[Non-Patent Document 1] K. Morito et. al. IEEE Journal of Lightwave Technology, 2003, 21, (1), pp. 176-181.
[Non-Patent Document 2] S. Tanaka et. al. IEE Electronics Letters, vol. 42, no. 18, pp. 1059-1060, 2006.
[Non-Patent Document 3] K. Magari et. al. IEEE, Journal of Quantum Electronics, vol. 30, No. 3, 695-702 (1994)
However, in the former reported case, the flat gain wavelength characteristic (requirement B) in the wavelength band of 1.55 μm is not satisfied sufficiently. The following (1) and (2) are considered to be the reason.    (1) A band-edge wavelength of an active layer material (GaInAs (P)) shifts to a shorter wavelength side due to an effect of an tensile strain which is applied to the active layer material for realizing a polarization-independent amplification characteristic.    (2) When a current with high-density is injected into an SOA having a thin active layer in order to obtain the high saturation optical output power (requirement D), a gain peak wavelength shifts to the shorter wavelength side because of a band-filling effect caused in the active layer.
According to the effects of the above-described (1), (2), and the like, when the SOA is actually used, the gain peak wavelength shifts to the shorter wavelength side from the wavelength of 1.55 μm, resulting that a gain tilt (tilt according to a wavelength dependency of the gain) in which the gain becomes higher at the shorter wavelength side in the vicinity of the wavelength of 1.55 μm tends to be produced.